1146 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



These all add up to the general requirement: 



g«Go«G (17) 



and the particular choice of Go is generally a compromise between good 

 discrimination and low transmission loss. 



PEDESTAL 



A major defect of diode gates is an output voltage variation produced 

 by control voltage changes. The output voltage is very small and 

 negative when the gate is disabled, but when the control voltage switches 

 the gate to the enabled condition a positive voltage, called the pedestal, 

 appears at the output. From (5) and (G), with 7o = 0, it is 



F, = 1 ^^ + C^" (18) 



^°2 + | + l 



Since g is small, this approximates the voltage due to half the bias and 

 control currents flowing in the output. The signal output is superposed 

 on this pedestal and may swing from zero to twice V2. If the frequencies 

 involved in the signal and the control voltages are widely different, this 

 pedestal is not important since it can be filtered out from the transmitted 

 signal but it is a serious output distortion in other cases where signal 

 frequency components from the control pulse may be transmitted as a 

 spurious signal. 



EXPERIMENTAL CHECK 



In the preceding discussion it was tacitly assumed that a diode switches 

 between two constant conductance values. On a dc basis, the equations 

 are still valid with a variable conductance, except for the signal loss 

 relations. In computing the small signal loss, the conductances chosen 

 should be the dynamic or differential conductances at the particular bias 

 currents chosen. 



The following is an example of the kind of experimental checks ob- 

 tained in which, using 400B diodes, the actual behavior of a gate was 

 tested on a dc basis. The particular units had approximately an im- 

 pedance ratio (at about 5 volts) of 20,000/200. The load conductance 

 was chosen (somewhat arbitrarily) as the geometric mean of the diode 

 conductances, giving: 



G = 5 10"' 



^ = 5 10"' 



Go = 5 10"* 



